With the recent advancement of digital technologies, there is a strong tendency toward high speed processing or storage of high capacity data, and a demand for high integration and high performance of semiconductor devices used in electronic apparatus. In order to realize high integration of a semiconductor device, as a capacitor dielectric film of a capacitor which forms a storage element, a high dielectric constant material film or a ferro-electric material film is being used in place of a conventional silicon oxide film or a silicon nitride film.
As a nonvolatile memory capable of high speed read/write at a low voltage, a ferro-electric random access memory (FeRAM) in particular has been studied and developed vigorously which uses a ferro-electric film having spontaneous polarization characteristics as a capacitor dielectric film.
A ferro-electric memory (FeRAM) is a nonvolatile memory in which stored information will not be erased even if a power supply is shut down, and is expected to realize high integration, high speed driving, high durability and low power consumption.
FeRAM stores information by utilizing hysteresis characteristics of ferro-electric material. A ferro-electric capacitor having a ferro-electric film as a capacitor dielectric film sandwiched between a pair of electrodes generates polarization corresponding to a voltage applied across the electrodes, and retains the polarization even after the applied voltage is removed. As the polarity of the applied voltage is reversed, the polarity of polarization is also reversed. By detecting this polarization, information can be read. As the material of a ferro-electric film, oxide ferro-electric material having the perovskite crystal structure is used mainly, such as PZT(Pb(Zr1-xTix)O3) and SBT(SrBi2Ta2O9) having a large polarization quantity, e.g., about 10 μC/cm2 to 30 μC/cm2. In order to form an oxide ferro-electric film having excellent characteristics, the film is required to be formed or to be subjected to heat treatment in an oxidizing atmosphere, and a lower electrode (also an upper electrode when necessary) is often made of noble metal hard to be oxidized, or noble metal maintaining conductivity even if it is oxidized or noble metal oxide.
Before a ferro-electric capacitor is formed, a MOS transistor is formed on a silicon substrate. When a ferroelectric capacitor is formed after the lower structure such as MOS transistors is formed, it is necessary that the oxidizing atmosphere during formation of a ferro-electric film should not adversely affect the lower structure. For example, after MOS transistors are formed, the MOS transistors are protected by a film such as a silicon oxynitride film having an oxygen shielding ability, and an interlayer insulating film is formed on the oxygen shielding film.
The interlayer insulating film of a semiconductor integrated circuit device is made of silicon oxide in many cases. Silicon oxide has high affinity with moisture. As moisture permeates from an external, moisture can reach wirings, capacitors, transistors and the like through the interlayer insulating film. As moisture reaches a capacitor particularly a ferro-electric capacitor, the characteristics of a dielectric film particularly a ferro-electric film are deteriorated. If the ferro-electric film is reduced by hydrogen derived from permeated moisture and oxygen defects are formed, crystallinity becomes bad. The characteristics are deteriorated such as a reduced residual polarization quantity and a lowered dielectric constant. Similar phenomena occur by long term use. As hydrogen permeates, deterioration of the characteristics becomes more direct than moisture. Silane used as silicon source for forming a silicon film or a silicon oxide film is silicon hydride, and generates a large amount of hydrogen when decomposed. Hydrogen is also a factor of deteriorating a ferro-electric film.
In a standard ferro-electric capacitor having a structure that a PZT ferro-electric film is sandwiched between lower and upper electrodes made of Pt, it is known that ferro-electricity of the PZT film is almost lost if the substrate is heated to about 200° C. in an atmosphere at a hydrogen partial pressure of 40 Pa (0.3 Torr).
It is also known that the ferro-electricity of the ferro-electric film is degraded considerably if heat treatment is performed on a ferro-electric capacitor in a state that hydrogen or moisture is absorbed in the capacitor or in a state that moisture exists near the capacitor.
In manufacture processes for FeRAM, processes after the ferro-electric film is formed are so selected that generation of moisture and hydrogen is as less as possible and the process temperature is low. For example, a silicon oxide film is formed by chemical vapor deposition (CVD) using tetraethoxysilane (TEOS) having a relatively small hydrogen generation amount, as Si source gas.
A process of forming a lower electrode just under the ferro-electric film is important in forming a ferro-electric capacitor. A conventional lower electrode has a structure that Ti and Pt are sequentially laminated on an insulating film. The Ti film improves adhesion between the insulating film and the lower electrode. If the Ti film is not used, there is a high possibility that the Pt electrode is stripped or peeled off. The Pt film is formed by sputtering. If this sputtering is performed at a high temperature, the Pt film reacts with the Ti film, and will not cause (1 1 1) orientation, resulting in a randomly oriented structure. If a TiO2 film is used instead of the Ti film, reaction is suppressed so that the Pt film can be formed at a high temperature. However, as a TiO2 film is formed on a degassed insulating film, crystallinity of the TiO2 film is degraded, and crystallinity of the Pt film and ferro-electric film formed on the TiO2 is lowered.
JP-A-2002-289793 (applicant: Fujitsu Limited), which is incorporated herein by reference, proposes to use a lamination structure of a TiO2 film on an SiO2 film, or an alumina film, as an insulating adhesion enhancing film under a Pt lower electrode.
JP-A-HEI-7-14993 (applicant: Mitsubishi Electric Corporation) proposes a DRAM semiconductor device using a high dielectric constant film such as SrTiO3. It is pointed out that when a lower electrode of a flat plane shape is formed on an interlayer insulating film of silicon oxide formed with an Si via conductor connected to a transistor and a high dielectric constant film such as SrTiO3 is formed on the interlayer insulating film, covering the lower electrode, the high dielectric constant film is likely to be peeled or stripped off from the interlayer insulating film, and proposes to form an insulating adhesion enhancing film between the interlayer insulating film and high dielectric constant film. The insulating adhesion enhancing film is made of TiO2, ZrO2, Ta2O5, Si3N4 or Al2O3. After the insulating adhesion enhancing film is formed on the whole surface of the interlayer insulating film, a polysilicon via conductor is formed, a Pt lower electrode film is formed on the insulating adhesion enhancing film, via a TiN barrier film which prevents silicide reaction, and patterned. Thereafter a high dielectric constant film is formed above the interlayer insulating film, covering the lower electrode, and an upper electrode layer common to a number of capacitors is formed on the high dielectric constant film.
JP-A-2005-39299 (applicant: Matsushita Electric Industrial Co. Ltd.) proposes, in a ferro-electric capacitor having a structure that a ferro-electric film covers a lower electrode formed on an interlayer insulating film and an upper electrode is formed on the ferro-electric film, to form a conductive hydrogen barrier film, covering the upper electrode and having an overhang portion extending over the interlayer insulating film. After an upper interlayer insulating film is formed covering the ferro-electric capacitor, a via hole is formed reaching the overhang portion of the conductive hydrogen barrier film, and a conductive plug is formed in the via hole. It teaches that it is preferable to use, as the conductive hydrogen barrier film, a Ti film, a Ta film, a TiON film, a TiN film, a TaN film, a TiAlN film, a TiAlON film or an alloy film containing these.
JP-A-2003-174146 (applicant: Fujitsu Limited), which is incorporated herein by reference, proposes to form an upper electrode with a lamination of two types of noble metal oxide films. Transistors formed on a semiconductor substrate are covered with an insulating barrier film having an oxygen shielding ability such as a silicon nitride film and an silicon oxynitride film in order to prevent an oxidizing atmosphere during formation of the ferro-electric film from adversely affecting the transistors. The ferro-electric capacitor is covered with an insulating barrier film having a hydrogen shielding ability such as alumina in order to prevent the characteristics of the ferro-electric capacitor from being degraded by heat treatment in a reducing atmosphere.